  | |
| Observation data Epoch J2000 Equinox J2000 | |
|---|---|
| Constellation | Aquila |
| Right ascension | 19h 05m 24.60802s[1] |
| Declination | +13° 51′ 48.5182″[1] |
| Apparent magnitude (V) | 2.983[2] |
| Characteristics | |
| Spectral type | A0 Vn[3] |
| U−Bcolor index | +0.080[2] |
| B−Vcolor index | +0.009[2] |
| Astrometry | |
| Radial velocity (Rv) | −25[4] km/s |
| Proper motion (μ) | RA: −0.25mas/yr[1] Dec.: −95.56mas/yr[1] |
| Parallax (π) | 39.28±0.16 mas[1] |
| Distance | 83.0 ± 0.3 ly (25.5 ± 0.1 pc) |
| Absolute magnitude (MV) | +0.96[5] |
| Details[6] | |
| A | |
| Mass | 2.53±0.16 M☉ |
| Radius | 2.762 (equatorial) 2.148 (polar) R☉ |
| Luminosity | 52.5+2.5 −2.4 L☉ |
| Surface gravity (log g) | 3.60 (equatorial) 4.15 (polar) cgs |
| Temperature | 8,680 (equatorial) 10,952 (polar) K |
| Metallicity [Fe/H] | −0.52±0.04[5] dex |
| Rotation | 10.575 hours |
| Rotational velocity (v sin i) | 306+20 −5 km/s |
| Age | 100±50[7] Myr |
| B | |
| Mass | 0.50[8] M☉ |
| Other designations | |
| Okab (primary),Zeta Aql, ζ Aql,17 Aql,BD+13 3899,FK5 716,GJ 4095,HD 177724,HIP 93747,HR 7235,SAO 104461,WDS J19054+1352A[9] | |
| Database references | |
| SIMBAD | data |
Zeta Aquilae, orζ Aquilae, is abinary star[8] system in the equatorialconstellation ofAquila. It is readily visible with the naked eye, being of the third magnitude.[2] Based onparallax measurements obtained during theHipparcos mission, it is approximately 83light-years (25parsecs) distant from theSun.[1] It is a candidate member of theTW Hydrae association of co-moving stars.[10]
Zeta Aquilae's two components can be designated Zeta Aquilae A (officially namedOkab/ˈoʊkæb/, the traditional name for the system)[11] and B. Zeta Aquilae has a number of companions listed and together they are designated WDS J19054+1352. As the primary star of this group, Zeta Aquilae also bears the designation WDS J19054+1352A.[9] The companions are then designated WDS J19054+1352B, C, D and E.[12][13][14][15]
ζ Aquilae,Latinised toZeta Aquilae, is the binary'sBayer designation. The designations of the two components asZeta Aquilae A andB derive from the convention used by the Washington Multiplicity Catalog (WMC) formultiple star systems, and adopted by theInternational Astronomical Union (IAU).[16] WDS J19054+1352 is the entry of the wider system of which Zeta Aquilae is a member in theWashington Double Star Catalog.
Zeta andEpsilon Aquilae together bore the traditional nameDeneb el Okab, from anArabic term ذنب العقابDhanab al-ʽuqāb "the tail of the eagle", which they mark (Aquila isLatin for 'eagle').[17] In 2016, the IAU organized aWorking Group on Star Names (WGSN)[18] to catalog and standardize proper names for stars. The WGSN decided to attribute proper names to individual stars rather than entiremultiple systems.[19] It approved the nameOkab for the component Zeta Aquilae A on 1 June 2018 and it is now so included in the List of IAU-approved Star Names.[11]
Epsilon and Zeta Aquilae also bore theMandarin namesWoo/ˈwuː/ andYuë/ˈjuːeɪ/, derived from and representing the old statesWú (吳) (located at the mouth of theYangtze River) andYuè (越) (inZhejiang province).[17][20]
In the catalogue of stars in theCalendarium of Al Achsasi Al Mouakket, Zeta Aquilae was designatedDzeneb al Tair (from the Arabic ذنب الطائرðanab aṭ-ṭā’ir), which was translated into Latin asCauda (Vulturis) Volantis, meaningthe eagle's tail.[21]
InChinese,天市左垣 (Tiān Shì Zuǒ Yuán), meaningLeft Wall of Heavenly Market Enclosure, refers to anasterism which represents eleven old states in China and is marking the left borderline of the enclosure, consisting of Zeta Aquilae;Delta,Lambda,Mu,Omicron and112 Herculis;Theta¹ andEta Serpentis;Nu Ophiuchi,Xi Serpentis andEta Ophiuchi.[22] Consequently, theChinese name for Zeta Aquilae itself is天市左垣六 (Tiān Shì Zuǒ Yuán liù, English:the Sixth Star of Left Wall of Heavenly Market Enclosure), representing the state mentioned above.[23]
The primary, designated component A, has astellar classification of A0 Vn,[3] with theluminosity class 'V' indicating is amain sequence star that is generating energy through thenuclear fusion of hydrogen at its core. It has more than doublethe mass and twicethe radius of theSun,[24][7] and is radiating more than 39 timesthe Sun's luminosity.[7] Theeffective temperature of the star's outer envelope is about 9620 K,[7] which gives it the white hue typical ofA-type stars. The estimated age of this star is 50–150 million years.[24]
This star is rotating rapidly, with aprojected rotational velocity of 306 km s−1 giving a lower bound on theazimuthal velocity along the equator. As a result, it has an oblate shape, with its equatorial radius measuring 2.76 solar radii (R☉) and its polar radius measuring 2.15 R☉.Gravity darkening due to the fast rotation also make itseffective temperature andsurface gravity vary across latitudes, from8,680 K and3.60 cgs in the equator to11,000 K and4.15 cgs in the poles.[6] Because of theDoppler effect, this rapid rotation makes theabsorption lines in the star'sspectrum broaden and smear out, as indicated by the 'n' suffix in the stellar class. It likely exhibitsdifferential rotation along different latitudes. It is rotating nearly edge-on relative to Earth, with an inclination of 85°.[6]
Astronomers use Zeta Aquilae as a telluric standard star.[25] That is, the spectrum of this star is used to correct fortelluric contamination from the Earth's atmosphere when examining the spectra of neighboring stars.[26] Observation of this star in theinfrared band during the2MASS survey appeared to revealexcess emission. However, the distribution of this emission couldn't be readily explained by a conjectureddisk of circumstellar dust.[24] Instead, the detection was later ascribed to errors caused by saturation of the near-infrared detectors.[7]
The primary forms a binary star system with component B.[8] This is a magnitude 12.0 star at anangular separation of7.20″ along aposition angle of 46°, as of 2009.[27] The pair have aprojected separation of185.1 AU. The secondary has an estimated mass equal to one half the mass of the Sun.[8] The 16th magnitude star WDS J19054+1352E is also considered to be a co-moving companion with a mass of 0.14 M☉, at a projected separation of38,000 AU from the primary.[8]
TheWashington Double Star Catalog lists a second 12th magnitude star at 160" (WDS J19054+1352C) plus an 11th magnitude star separated by 200" (WDS J19054+1352D).[27] TheCatalog of Components of Double and Multiple Stars lists the two 12th magnitude companions at 6.5" and 160".[28]
